In related arts, a technology is known by which an optical signal-to-noise ratio (OSNR) is analyzed based on an optical signal received by a transfer device in an optical network in order to detect malfunction and failure. For example, there is a technology in which filtering is performed, by a narrow band optical filter, on an optical signal on which superimposition of AM modulation is performed in the vicinity of the signal central wavelength and at a point that is shifted from the signal center wavelength by a certain wavelength portion, and an OSNR is measured from a ratio of a direct current (DC) component to an alternating current (AC) component after photoelectric conversion (for example, United State Patent Application Publication No. 2012/0106951). In addition, there is a technology in which an OSNR is measured from a ratio of peak power to trough power in light that is permeated through an optical filter having a certain permeation characteristic (for example, X. Liu, et al. “OSNR Monitoring Method for OOK and DPSK Based on Optical Delay Interferometer”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 15, Aug. 1, 2007, Pp. 1172-1174).
However, in the above-described related arts, a configuration for measuring an OSNR is complicated.